Resistance to the important beta-lactam antibiotics, like penicillins and cephalosporins, due to the production of extended spectrum beta-lactamases is increasingly found in bacterial infections in humans outside the hospital. This could lead to increasing numbers of difficult to treat bacterial infections. The source of these bacteria is unknown, but consumption of food could be one of the sources.
In her thesis Cindy Dierikx, veterinary microbiologist in training at the Central Veterinary Institute (CVI), part of Wageningen UR, describes the presence of these resistant bacteria in broilers. Dierikx defends her thesis May 30 at the Veterinary Faculty of Utrecht University.
Since 2003 beta-lactam resistance has increased bacteria from broilers. This increase is observed in the national monitoring of antibiotic resistant bacteria from food-producing animals. The beta-lactam resistant bacteria are ubiquitous present in the broiler production pyramid, especially in broiler farms.
Bacteria become resistant to beta-lactam antimicrobials by producing certain enzymes that inactivate these antimicrobials. The two main groups of enzymes are the extended spectrum beta-lactamases (ESBLs) and AmpC beta-lactamases. The genes encoding these enzymes are located on mobile genetic elements, named plasmids. These plasmids and the associated resistance genes, can easily be exchanged between bacteria, causing spread between bacteria - not only between bacteria of the same species but also between different bacterial species.
The mechanism by which bacteria develop resistance in broilers is similar to what is found in bacteria causing urinary tract infections in people. In addition, 94% of broiler meat purchased at different stores in the Netherlands contained similar ESBL-producing bacteria. One in five of ESBL-producing E. coli isolates from human infection is genetically related to isolates from poultry. The transfer via poultry meat to humans is considered a likely transmission route.
Approximately 10% of people carry ESBL-producing bacteria in their intestines; for broiler farmers this percentage is 33%. ESBL producing isolates were also found among clinical infections in companion animals and horses. ESBL genes found in these isolates are partly similar to the ones found in broilers, but also include types found more often in humans, which shows another possible route of these bacteria to humans.
Dierikx states that it is advisable to take measures to reduce ESBL-producing bacteria in the broiler production pyramid. In recent years much has been accomplished by reducing the amount of antimicrobials. The broiler industry is faced with the challenge to make a healthy, safe and animal friendly produced product, where antibiotics are hardly necessary.